1. Field of the Invention
The present invention relates to a variable impedance circuit and apparatus comprising said variable impedance circuit.
2. Description of the Related Art
Resistive circuits are generally known in which resistance varies linearly; said circuits are, for example, used in devices provided with amplifiers in which it is necessary to obtain a linearly variable gain.
The simplest way to obtain an impedance that varies linearly is to use an MOS transistor operating in a triode zone; nevertheless, if a wide impedance variation is necessary it is not possible to use a simple MOS transistor.
A circuit that enables a wide resistance variation to be obtained is available with the circuit in FIG. 1. In said circuit a resistor Rp is arranged between an input voltage Vinn and ground GND and the terminals thereof are connected to the drain and source terminals of a NMOS transistor M1p having the gate terminal connected to a voltage Vconn. The value of the resistance of the entire circuit increases when the voltage between gate and source of the transistor M1p decreases, i.e. when the transistor moves away from the triode region of operation. This latter situation gives rise to a total non-linear resistance as the resistance of MOS transistor M1p, which in this case is the predominant resistance, varies in a non-linear manner due to the passage of the transistor from the triode region of operation to the saturation region of operation during the swing of the signal Vinn. When on the other hand the resistance of the resistor Rp is predominant, total resistance becomes linear.
Another circuit that enables wide resistance variation to be obtained is the resistor with controlled voltage of patent WO 03/021777. Said resistor comprises an input gate and an output gate and at least two circuit blocks arranged between said two gates. A circuit block has a first impedance that is variable and controllable by means of a first signal and the second circuit block has a second impedance that is variable and controllable by means of a second signal; the two impedances can be controlled in an independent manner. Total impedance is arranged so as to be variable in a continuous manner within a given variation field of the values of the control signals.